1. Field of the Invention
This invention relates to an apparatus to distribute stresses imposed on a threaded fastener apparatus over a length of mating threads to more effectively transfer an applied force which may cause elastic deformation of interfitting threads over the length of thread of two members which are in mutual engagement.
2. Description of the Prior Art
Conventional screw thread systems have been developed through the use of various screw thread forms most of which have isometrical sides inclined at equal angles with a vertical center line through the thread appex. Examples of present day thread forms include the Unified and the Whitworth V-thread forms. An example of earlier thread forms which are used occasionally is the Sharp V-thread form. Isometrical V-threads are relatively easy to manufacture and inspect; hence they are widely used on mass produced, general-purpose threaded fasteners of all types. In addition to general-purpose fastener application, certain threads are used to repeatedly move or translate machine parts against heavy loads. For these so-called translation threads a more efficient thread form is required. The most widely used thread forms for this purpose are the Square, the Acme and the Buttress. The Square thread is generally regarded as most efficient, but is difficult to cut because the thread form provides parallel sides. It also cannot be adjusted to compensate for wear. The Acme form of thread does not suffer from the disadvantages of the Square thread form; it is stronger and only slightly less efficient. The Bustress thread form is used for translation of loads in one direction only. Because of its non-symmetrical form, it combines the high efficiency of the Square thread with the high strength of the V-thread and with the ease of cutting and adjustment of the Acme thread. Translation thread forms are usually only loaded slightly and the application of a force is not sufficient to cause elastic deformation of one or more interfitting threads.
The applied load can be either a tension load or compressive load on the externally threaded member with respect to the internally threaded member.
To achieve a uniform loading of the externally threaded member over a given threaded length, it is necessary that the elastic deformation over that length be sufficient to create the desired stress between all of the mating threads. There are, however, many kinds of elastic deformations within the threaded connection whereby an exact analysis becomes very complex. For example, there may be slack in the joint of the workpiece being acted upon by the threaded fastener; slack between mating threads of the fastener; elastic deformation of the workpiece and/or fastener; surface roughness on the workpiece and/or fastener; a tolerance mismatch; and bending of threads. However, for practical purposes, one can make assumptions that will render practical results. A practical approach is to derive a measure of a unit strain which must be accommodated by the threads with the elastic limit of the material comprising the threads. A simple formula for unit strain defines that the strain is equal to the stress which must be accommodated by the thread system divided by the modules of elasticity. For example, if one desires to accommodate a stress of 90,000 pounds per square inch in a fastener having a modules of elasticity of 30,000,000 pounds per square inch, the unit strain is calculated to equal 0.003 inches per inch. This means that if the fastener is loaded over a length of one inch, the fastener will have to be stretched 0.003 inch within the one inch of thread length. If the fastener were loaded over a thread length of 10 inches, the fastener will be stretched by an amount equal to 0.030 inch along the thread length of 10 inches.
External and internal thread forms traditionally have the same thread pitch to transfer loads between the standard internal and external threads. Because of the elastic properties of the material comprising the thread, a concentration of high loads and stresses occur at the site where the applied force is initially transferred from one member to the other. Because of the relative rigidity of the thread materials, the initial convolutions of mating screw threads will transfer most of the forces resulting in a high concentration of stresses at this site. Elastic and plastic deformations of the first convolution of mating threads allow a certain amount of forces to be transferred by subsequently occurring thread convolutions. Thus, in actual practice, all of the forces are transferred by only a few of the mating thread convolutions.
The elastic deformation of the material forming the threads at the initial site for transfer of loads leads to static or fatigue failure of the materials, thus, also a failure of the screw thread system. This invention seeks to avoid this problem by utilizing a thread system in which internal and external threads forming a differential thread pitch are brought into mating engagement in a manner so that the tooth flanks of the elastically interfitting threads move through clearance distances in a progressively increasing manner in response to forces causing the tooth flanks to mate, depending upon the magnitude of the applied force and transmitted force. Moreover, it can be seen that the selection of the differential pitch can be made in relation to the member receiving the applied force so that the progressive establishment of a force transmission between the tooth flanks proceeds in the same direction as the applied force. It is necessary, however, according to the present invention that interference does not exist between the threads along the entire length of the interfitting threads. The present invention is particularly advantageous when utilized to mechanically stress a bolt-type fastener as disclosed in my U.S. Pat. No. 4,622,730.
As shown, for example, in U.S. Pat. No. 4,266,590 a uniform pitch of nut thread is a predetermined amount less than the uniform pitch of bolt thread so that within the threaded length of the nut there is flank-to-flank contact between threads at the top and at the bottom of the nut. The flank-to-flank contact between the screw threads at opposite ends of the nut must transmit at only one site all the forces which must be transferred between the nut and screw of the fastener. The mating screw threads between the opposite ends of the nut come into flank-to-flank engagement only after the mating thread flanks distort at one end of the fastener by an amount sufficient to alter the spacing between the subsequently occurring thread flanks. Another form of differential thread pitch for a fastener device can be found in a differential pitch to the threads on a fastener in which the pitch of the screw threads varies along the length of the threads. An example of such a threaded fastener is shown in U.S. Pat. Nos. 3,454,070; 3,799,299; and 2,356,098. Lastly, U.S. Pat. No. 1,869,156 discloses the combination of a screw thread in one member having a uniform pitch and a screw thread in a second member having a different and variable pitch so that the threads will frictionally lock together the co-acting threaded members. In all these known fasteners, the threads of at least one member are jammed or upset by the threads of the other member in a manner which can be generally characterized by the fact that a tension or compression force on one of the members must always be taken by only a minor portion of the mating length of threads.